1. Technical Field
This invention is directed to a process for fabricating integrated circuit devices in which a layer of an oxynitride material is formed.
2. Art Background
Semiconductor devices such as MOS (metal-oxide-semiconductor) devices are typically formed on a substrate such as a silicon wafer. Insulating films of silicon dioxide are formed on the substrate over which is formed a gate electrode. The insulating film formed between the gate electrode and the silicon substrate is referred to as the gate oxide, or gate dielectric.
The incorporation of nitrogen into the gate oxide is advantageous because it improves certain characteristics of the oxide, such as the charge-to-breakdown of the oxide and its resistance to hot carrier damage. The presence of nitrogen in the gate oxide also acts as a barrier against boron diffusion from the overlying layers of doped polysilicon into the underlying silicon substrate.
Various methods for the incorporation of nitrogen into silicon dioxide have been proposed. Currently, a silicon wafer is heated in an atmosphere of either N.sub.2 O or NO for a period of time that is adequate to incorporate the desired amount of nitrogen into the growing silicon dioxide film. This may or may not be followed by further oxidation of the wafer in an O.sub.2 atmosphere, depending on the desired thickness of the resulting film.
Processes for forming silicon oxynitride have been the subject of considerable investigation. Saks, N. S., et al., "Nitrogen depletion during oxidation in N.sub.2 O," Appl. Phys. Lett., Vol. 67(3), pp. 374-376 (1995) describes tests in which N.sub.2 O-oxidized silicon wafers are placed in an atmospheric oxidation furnace at 900.degree. C. After about five minutes in an O.sub.2 atmosphere, the wafers were re-oxidized in pure N.sub.2 O. Saks et al. observe that nitrogen is incorporated into the oxide at lower temperatures (900.degree. C.) in an N.sub.2 O atmosphere than the temperature at which nitrogen is incorporated into the silicon in an N.sub.2 atmosphere. However, Raider, S. I., et al., "Nitrogen reaction at a silicon--silicon dioxide interface," Appl. Physics Letters, Vol. 27(3), pp. 150-152 (1975) observes that continuous nitride films do not form to any significant thickness at temperatures less than about 1200.degree. C.
As observed by Saks et al., the NO species formed by the breakdown of N.sub.2 O at the furnace temperature depletes nitrogen from the bulk of the oxide, resulting in an unequal distribution of nitrogen in the oxide, with the bulk of the nitrogen concentrated at the interface between the oxide and the underlying substrate. Since nitrogen acts as a barrier to boron diffusion, the nitrogen content at the oxide/substrate interface does not prevent boron from penetrating the oxide, but, rather, merely prevents the boron from diffusing further into the underlying substrate. The presence of boron in the oxide will degrade oxide reliability. Therefore, a process that provides a more uniform distribution of nitrogen in the silicon oxynitride layer is desired.
Furthermore, the current processes for forming silicon oxynitride utilize atmospheres of nitrous oxide (N.sub.2 O) or nitric oxide (NO). These gases are expensive and both of them are asphyxiants, so their use must be carefully controlled. Therefore, a process that utilizes a less expensive source of nitrogen is desired.